In the synthesis of various carotenoids, mixtures of cis- and trans-isomers are typically produced. In general, the all-trans-isomer of a carotenoid is much more valuable than any of the cis-isomers, and, for many carotenoids, the all-trans-isomer is the only isomer of any commercial value. For this reason, with respect to a large number of carotenoids, ways have been sought to convert cis-isomers to the corresponding all-trans-isomer.
The methods found for converting cis-carotenoids to all-trans-carotenoids have not been entirely satisfactory. See for example U.S. Pat. No. 2,849,507 and U.S. Pat. No. 3,441,623. Note also Karrer and Jucker, Carotenoids, Elsevier Publ. Co., Inc., Amsterdam, pp. 38-42 (1950) and Helv. Chim. Acta., 39, 249 (1956). In U.S. Pat. Nos. 2,849,507 and 3,441,623, cis-.beta.-carotene is converted to all-trans-.beta.-carotene by heating the cis-isomer in an inert organic liquid. However, the all-trans-.beta.-carotene obtained by the processes of these patents invariably contains at least a small amount of cis-isomers as contaminants. The residual cis-isomers are difficult to remove and render the .beta.-carotene obtained unsatisfactory for human consumption unless removed.
There has been an unfilled need, therefore, for a generally applicable method for completely converting cis-isomers of a carotenoid to the corresponding all-trans-isomer without the need for carrying out extensive purification procedures to removal residual cis-isomers.